In many buildings, especially those that are prefabricated, joists are used instead of I-beams. However, while joists are lighter and cheaper than solid beams, they are more flexible. When they are used to support floors which are intended to bear machines or heavy objects, vibrations or oscillations are set up when the machines are operated or the heavy objects are moved over the floors. Furthermore, building structures are subjected to natural forces such as wind, earth tremors caused by passing traffic, or the like. Various methods have been proposed to meet the problem of vibrations. I have discovered a simple and extremely efficient means of using comparatively light joists extending over relatively long spans and preventing damage and annoyance caused by vibrations by damping them.